Historically, lithographic web offset heatset inks contain between 30% and 45% volatile organic compounds (VOC). Besides being detrimental to the environment, VOCs are also flammable and hazardous to the printers who operate the press. Thus, it is desirable to reduce the VOC content in lithographic web offset heatset inks as much as possible. Initial attempts at solving this problem involved the use of chemical reactions that were triggered in a press oven. However, such oven cured ink systems did not have shelf stability.
Therefore, a heat setting web offset ink will typically contain the following major components (a) a high molecular weight ink resin to disperse the pigment and also to provide the toughness and gloss the ink requires on drying, (b) solvents to provide the fluidity to the ink before it is placed on the web and dried in an oven, (c) pigment, and (d) other minor components such as gellants, which provide structure to the ink, plasticizers (non volatile solvents), waxes, thickeners, and antioxidants. Conventional heatset inks set or dry by evaporation of the ink oil on heating at 250–300° F., and, to some degree, by penetration of the ink oil into the paper, leaving behind a hard polymeric film.
EP 731150 A1 960911 describes rapid thermosetting low VOC web offset lithographic ink systems comprising solid resin, drying oil alkyds, bodied drying oil, vegetable oil, fatty acids, multifunctional unsaturated polyester, reducing agents and transition metal salts of organic acids and may also include an aqueous fountain solution containing peroxides that promote free radical polymerization of the ink. WO96/34922, U.S. Pat. No. 5,431,721, and U.S. Pat. No. 5,545,741, 1996 respectively describe lithographic inks which employ non-volatile solvents, but they set by penetration of the non-volatile solvent into the stock.
Thus, there is still a need to formulate low VOC web offset heatset inks that have good shelf stability and high dry speed.